FIG. 1 shows timing diagrams indicating transmitted/received signals and the principle of mixing processes in the FM-CM radar technique. FIG. 2 is a plan view illustrating an example of an environment of a road with opposite lanes. FIG. 3 shows by (a) and (b) conditions of signal processing in a radar system equipped in a vehicle in the case where an interference signal has been received from another vehicle.
In order to prevent collisions between vehicles and to perform inter-vehicle control, on-vehicle radars have been developed, which measure a distance, speed and azimuth associated with a front target, such as a preceding vehicle.
As a means for measuring a distance from a front target and a relative speed of the front target, the FM-CW radar technique has been used because, for example, this technique uses a signal processing circuit having a simplified configuration. Also, for measuring an azimuth, the electronic scanning technique has been used (e.g., refer to Patent Document 1).
In the FM-CW technique, a transmission wave Tx whose frequency change linearly as shown by (a) of FIG. 1 is horizontally transmitted over a predetermined angle in a fan-like form from a transmitting antenna, as shown in FIG. 2, centering on a vehicle center CL of a vehicle 51. The transmission wave Tx transmitted in a fan-like form is reflected by a target (e.g., oncoming vehicle 52), for the reception of a reflected signal Rx1 to thereby perform scanning of the fan-like region. Then, the reflected signal, or the reception signal Rx1, is mixed with the transmission signal Tx. This mixing produces a beat signal S, shown by (b) of FIG. 1, having a component of a frequency difference (beat frequency fb) between the transmission and reception signals. Conversion is carried out to obtain the distance using the fact that the frequency of the beat signal S is in proportion to a round-trip propagation delay time Δt for the target.
The techniques for measuring azimuth include the electronic scanning technique mentioned above, which enables scanning processes in all the azimuths in a short time. In the electronic scanning technique, a reflected wave from an object is received by a plurality of antenna elements (array antennas) arranged according to a certain rule. A time difference is caused between channels of received data, which time difference is determined by an azimuth α between a target and each antenna, an arranged position of each antenna, and frequency of a reception signal. Based on this time difference (or phase difference), an azimuth for the target can be detected. For example, digital beam forming (DBF) is known as a means for realizing such a technique. In the DBF, an azimuth is detected by digitizing a received data using an AD converter, and then correlating each channel with a vector data (mode vector) (e.g., refer to Non-patent Document 1).
In a road, as shown in FIG. 2, for example, which is busy with a number of vehicles equipped with radar systems, a radio wave Rx2 transmitted from a radar equipped in a vehicle 52 traveling along an opposite lane may be received by a radar equipped in another vehicle. Thus, the radio wave Rx2 from the oncoming vehicle (transmission wave from the oncoming vehicle) may interfere with a reflected signal Rx1 that is the reflection from a target, which is caused by the radio wave (transmission wave) Tx emitted from the other vehicle. In particular, the electrical power level tends to be high in a direct transmission wave emitted from each of the transmitting antennas of the radars loaded on other vehicles. In the case where the radars loaded on other vehicles use a modulation technique, such as the FM-CW technique, which involves a narrow-band signal, the electrical power level of the signal will be higher. The interference signal of a large electrical power level can be a major cause of the deterioration in the accuracy of measurement.
Under such circumstances, it will be effective to suppress the interference components contained in the reception signal. Methods have been suggested, for example, for suppressing the interference components by utilizing a filter for suppressing components coming from a particular azimuth (e.g., refer to Non-patent Document 2).
[Non-patent Document 1]                “Adaptive signal processing by array antennas” by Nobuyoshi Kikuma, Science and Technology Publishing Co., Ltd., 1998        
[Non-patent Document 2]                “Adaptive mainbeam jamming suppression for multi-function radars” by T. J. Nohara et al.        
[Patent Document 1]                Japanese Patent Laid-Open No. 11-133142        